Russian Soyuz flights are NASA's only way to get astronauts to and from the International Space Station. What happens if they cut us off?

With tensions escalating between Russia and Ukraine, the pressure is on President Obama to do more than issue stern warnings to the Russian government. Economic sanctions are one possible action, but one that could put the squeeze not only on Russia but also the U.S. manned space program.

Since the space shuttle retired in 2011, NASA has had no native human spaceflight capability. With no other options, NASA has relied on the Russian Federal Space Agency and its Soyuz rockets and spacecraft to get astronauts to and from the International Space Station, at a cost of tens of millions of dollars per seat. Any strong move by the U.S. in response to the Crimean crisis could spell the end of Americans flying on Russian spaceships, at least until tensions ease. NASA and its commercial partners have some projects in the works that can fill the gap, should Russia refuse to fly our astronauts. But these are at least two to three years from operational status. Depending on how the Russian-Ukraine crisis develops, those could be two to three years with no Americans in space.

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At the moment, most of NASA's human spaceflight resources are focused on the government-owned Space Launch System, or SLS. This was conceived as a deep-space rocket and spacecraft designed to send humans beyond low Earth orbit for the first time since the last astronauts left the moon in 1972. Although not specifically intended to send crews and supplies to the International Space Station, it could do so if necessary.

Unfortunately, SLS, which is consuming about $3 billion of NASA's annual budget, won't be ready to fly crew until 2021. And after that, it will be able to fly missions only once every four years under the current development schedule. So the multiple flights per year needed to maintain the International Space Station won't happen with SLS unless things change.

NASA's commercial crew program is a more promising alternative for flying Americans into space in American-built spacecraft. So far, NASA has split about $1.4 billion between a number of private companies trying to develop new manned spaceships (not counting the money spent on the cargo program that was the crew program's precursor). The companies receiving money under a program called Commercial Crew Integrated Capability, or CCiCap, are SpaceX, Boeing, and Sierra Nevada Corporation.

SpaceX is the leader of the pack, having already sent cargo to the ISS. SpaceX engineers designed the Falcon 9 rocket and Dragon space capsule with crew in mind from the beginning, and the company is now working with NASA to add crew accommodations and escape rockets to the Dragon. Boeing is at work on a crew capsule called the CST-100, while Sierra Nevada's Dream Chaser is the most shuttle-like of the commercial vehicles in development—its lifting body shape makes it steerable as a glider when returning to Earth. The current schedule has NASA choosing one or two of these companies this summer to complete a working crew-carrying spacecraft by 2017.

The big question is: Could NASA accelerate this schedule if it had to? We asked Jeff Foust, an analyst with the Futron Corporation and a long-time observer of the commercial spaceflight industry. "I don't think you would be able accelerate it that much," he says, "maybe 2016, maybe 2015. That's getting into the timeframe when the companies are anticipating their initial test flights... So if things got shut off now, there would still be an extended period of time when you wouldn't be able to do flights to the ISS."

Henry Hertzfeld, Professor of Space Policy and International Affairs at the George Washington University Space Policy Institute, expressed optimism that the situation won't come to that. "On the positive side," he says, "the ISS is an international partnership of 14 nations that includes Russia. Between that agreement and other space agreements that stress cooperation and peaceful uses of space, we and Russia may put a higher priority on that arrangement and access for all astronauts may be spared any direct impact from the political problems. I'm not making predictions, but at least we can hope that will happen."